I. Field of the Invention
The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and satellite repeater type spread spectrum communication systems. More particularly, the invention relates to a method and apparatus for establishing or maintaining signal diversity and improving signal acquisition for communication system subscribers in an urban environment by producing pseudo-multipath signals and indirect signal paths.
II. Description of the Related Art
A variety of multiple access communication systems have been developed for transferring information among a large number of system users. Techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, the basics of which are well known in the art. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in communication systems is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters," and U.S. patent application Ser. No. 08/368,570, filed under the title "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," both of which are assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose communication systems in which a large number of generally mobile or remote system users or subscriber units employ transceivers to communicate with other system users, or desired signal recipients such as through a public telephone switching network. Communication signals are transferred through satellite repeaters and gateways or terrestrial base stations (also sometimes referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals.
Using wide band CDMA techniques permits problems such as multipath fading to be more readily overcome, especially for terrestrial repeaters. CDMA signal processing provides a relatively high signal gain which allows spectrally similar communication signals to be more quickly differentiated. This allows signals traversing different propagation paths to be readily distinguished from each other, provided their respective path length differential causes relative propagation delays in excess of the chip period of the spreading code sequence used. Generally path length differentials of approximately 1 microsecond or more are easily distinguished, and typical urban environments provide differential paths delays of 10-20 microseconds.
The ability to discriminate between multipath signals greatly reduces the severity of multipath fading but typically does not totally eliminate it because of occasional paths with very small delay differentials. The existence of low delay paths is more especially true for satellite repeater based communication systems where multipath reflections from buildings and other terrestrial surfaces is reduced. Therefore, it is desirable to provide some form of signal diversity to reduce the deleterious effects of fading and additional problems associated with relative user, or repeater, movement.
Generally, three types of diversity are used in spread spectrum communication systems, and they are time, frequency, and space diversity. Time diversity is obtainable using data repetition, and time interleaving of data or signal components. A form of frequency diversity is inherently provided by CDMA in which the signal energy is spread over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links with a mobile or remote user through two or more base stations or antennas, for terrestrial-based repeater systems; or two or more satellite beams or individual satellites, for space-based repeater systems. That is, in the satellite communication environment or for indoor wireless communication systems, path diversity may be obtained by deliberately transmitting or receiving over multiple signal paths using multiple repeaters or transceivers. Furthermore, path diversity may be obtained by exploiting a natural multipath environment by allowing a signal arriving over different paths, each with a different propagation delay, to be received and processed separately for each path.
Examples of using path diversity in multiple access communication systems are illustrated in U.S. Pat. No. 5,101,501 entitled "Soft Handoff In A CDMA Cellular Telephone System," issued Mar. 31, 1992, and U.S. Pat. No. 5,109,390 entitled "Diversity Receiver In A CDMA Cellular Telephone System," issued Apr. 28, 1992, both assigned to the assignee of the present invention, and incorporated herein by reference.
Communication systems using satellites often maintain communication links for a mobile user through two or more satellites at any time to provide robust signal diversity. As disclosed above, such signal diversity helps combat problems with multipath fading which is intensified when operating in heavily industrial or high density urban environments. While signal diversity can overcome much of this problem, the situation is complicated by the fact that only one satellite may be "in view" of a subscriber unit or user in some circumstances.
Many modern urban areas have a large number of multi-story buildings or tall structures that might block signals from being received by or from one or more satellites that are normally "in view" of subscribers, but for the blockage. When these structures are grouped closely together, they form what is often referred to as "urban canyons". It can be seen that such urban canyons, or even single buildings, not only contribute greatly to multipath fading because of reflections, but also severely attenuate or completely block some signal paths to subscriber units. This prevents an appropriate use of multiple satellite beam signal diversity in some situations, greatly degrading system performance relative to certain users. In addition, some canyons might even totally block all direct `line-of-sight` paths to satellites.
Even when there appears to be a general line-of-sight communication path, there is also the matter of related structural interference. That is, users trying to establish either diversity or direct path communication links also have to deal with interference or severe signal attenuation from building materials when positioned interior to buildings, or from suspended building decorations, etc.
What is needed is a technique or apparatus that maintains or enhances signal diversity in urban environments that are prone to signal blockage. It is also desirable that such a technique require little complexity and control on the part of gateways, and can be directed to reach building interiors as well.